This invention relates to the drilling of a well into the earth and more particularly to treating the well to form a plug therein to control an active zone penetrated by the well.
In the drilling of a well into the earth, there is commonly employed a drill string having a drill bit connected to the lower end thereof. The drill string is rotated to rotate the drill bit and form a well or wellbore in the earth by cutting and breaking the earth formation which it contacts. Drilling fluid is conventionally circulated down the drill string and through ports provided in the drill bit and back to the surface through the annulus formed between the drill string and the wall of the well. The circulating drilling fluid performs numerous functions including removing the cuttings from the well, cooling the bit, and applying hydrostatic pressure upon the penetrated earth formation to control fluids contained under pressure therein. "Drilling muds" having either an oil base or a water base are commonly utilized as drilling fluids. These drilling muds are normally treated to provide desired density and rheological properties which make them particularly suitable for use in drilling wells. For example, drilling muds may be treated to increase the density thereof by adding thereto such materials as barium sulfate (barite), lead sulfide (galena), and iron oxide (hematite).
In the drilling of a well, a high pressure formation commonly referred to as an over-pressured formation is sometimes penetrated which contains formation fluids such as oil, gas and water under unusually high formation pressure. When the formation pressures exceed the mud column pressure in the well the formation fluids will flow into the well and push the mud from the well. Such a flow of formation fluids into the well is commonly called a "kick". If the kick is not controlled for example by closing blowout preventers provided in the drilling equipment to shut in the well, the formation fluids will push all of the mud from the well (blow out). After closing the blowout preventer, if the formations exposed to the well stand the kick the well may be brought under control by increasing the density of the drilling mud in the well to increase the hydrostatic pressure of the mud column to an amount sufficient to control the flow of fluids from the high pressure formation into the well. After increasing the density of the drilling mud, drilling operations may be continued and the well extended further into the earth.
Oftentimes in the drilling of a well, a weak formation commonly referred to as a "lost circulation zone" is penetrated by the well. This formation may be an active lost circulation zone into which unacceptably large amounts of drilling fluid are lost or may be a potential lost circulation zone. The active lost circulation zone may be treated to reduce the loss of drilling fluids thereinto to an acceptable level and drilling of the well may be continued. The potential lost circulation zone may or may not be recognized as it is penetrated by the wall. However, when an active zone is penetrated at a deeper level of the well and the well is shut in to prevent a blowout, the increased pressure due to formation fluids entering the well may result in breaking down the lost circulation zone such that drilling fluids are lost into the lost circulation zone and the active zone blows out underground into the lost circulation zone. Such conditions where a well simultaneously kicks and loses circulation are very dangerous.
In such a situation it may be desirable to place a plug in the well to control the active zone. In U.S. Pat. No. 3,490,535 to Joseph U. Messenger there is described a method of placing a plug within a well. An aqueous suspension of a weighting agent containing a water-soluble complex phosphate thinning agent in an amount within a range of 0.2 to 0.9 pound per barrel of water and exhibiting a pH within a range of 6 to 11 is circulated down a drill string within the well and into the wellbore externally of the drill string. Thereafter the drill string is withdrawn and the suspension is allowed to settle and form a plug which is resistant to high pressure differentials thereacross. Such a plug may be useful for controlling an active zone that is penetrated by a well wherein a lost circulation zone overlies the active zone. Another instance when it may be desirable to form a plug in a well is in conjunction with the making of a "trip", i.e. the withdrawal of the drill string from the well in order to change the drill bit. In making the trip, the well may be inadvertently swabbed as the drill string is withdrawn from the well with the attendant result that the hydrostatic head on the bottom of the hole is suddenly lowered. It may therefore be desirable to place a plug near the bottom of the hole before starting the trip such that should a swabbing action take place the possibility of a well blowout will be greatly reduced.